1. Field of the invention
The present invention relates to a device for measuring the passage of a predetermined length of cloth at right angles to the needle of a sewing machine comprising, on the one hand, a pair of reference points each forming an electrical contact member, an adjusting member for varying the distance between these reference points, these being intended to fix respectively the start and the end of the length to be measured, and, on the other hand, a contact closing member disposed between these reference points, a movable part fixedly associated with one of the contact members, the pair of reference points, or the contact closing member, means for putting this movable part in contact with the cloth to be sewn and for causing its movement as a function of the advance of this cloth, and a return member constantly tending to bring this movable part back into a position in which the reference point fixing the start of the length to be measured and the said contact closing member are applied one against the other.
2. Description of the prior art
Numerous devices associated with a control circuit of a sewing machine have already been proposed for measuring a predetermined length of cloth. The electrical signals indicating to the control circuit respectively the start and the end of the passage of a predetermined length of the cloth at right angles to the needle of the sewing machine may be generated by mechanical, magnetic, electro-optical or electrical means. The advantage of electrical means is in not having to depend on an intermediary and in acting directly on the electrical control circuit of the machine, this constituting an increase in precision and operating security which is not to be found with the other above-mentioned means.
Such a device has previously been proposed in U.S. Pat. No. A-4 409 913 for sewing buttonholes. For this purpose the bar of the presser foot is electrically conductive and is connected on the one hand to a terminal of the control circuit of the machine and, on the other hand, to a first contact element, secured to the presser foot. A second contact element, insulated from the first and also integral with the presser foot, is connected to the other pole of this control circuit by a wire passing through the interior of the tubular rod of this foot. These two contact elements are each provided with a sliding contact, which is in contact with an insulating track formed by two slide blocks, the first of which is slidingly mounted on the presser foot of the material presser and is adapted to be drawn along by the advance of the cloth. This first slide block carries a conductor disposed transversely to the insulating track, a second similar conductor being carried by the second slide block, which is adjustable by sliding on the first to determine the length of the path of the cloth. When the contact elements meet one or the other of the conductors, the two terminals of the control circuit are connected and the signal indicates that the measuring device is to be found at one or the other end of the length of the desired path for the sewing of the cloth.
In the solution proposed, the conductive reference points integral with the respective slide blocks are, from the electrical point of view, passive elements, as they are not directly intended to be connected to the control circuit, but are indirectly connected when the contact elements meet. This solution, in which the two contact elements are integral with the same element, necessitates insulating them from each other multiplying the number of parts necessary and increasing the assembly time. It is easily understood that this solution was adopted in the case where it would have been even more complicated to connect the reference points integral with the slide blocks to one of the terminals of the control circuit. Another inconvenience of this device resides in the fact that it is relatively cumbersome and that, to come into contact with the cloth, it is necessary to pinch the portion of cloth to be sewn, which complicates the work of the operator.